192 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Small crystals of pyrite are distributed throughout the cement and the fragments, 
and carbonates have also developed abundantly in both. There is a moderate 
amount of very fine aggregates of sericite. No secondary adularia could be detected. 
Partial analysis of altered breccia from the Golden Cycle mine. 
[W. T. Schaller, analyst.] 
Iv 2 0. 
2.79 CO, 
1.06 
3.85 
Si0 2 .. 54.57 
CaO. 
MgO. 
Na 2 0. 
S (total). 
7. 50 
3.42 
1.29 
This may be recalculated to Si0 2 , 54.57; CaC0 3 , 4.97; MgC0 3 , 2.23; FeC0 3 , 
0.17; FeS 2 , 2.42; Na 2 0, 3.85; Iv,0, 7.50, and shows that, though the substitution of 
K,0 for Na 2 0 has not been complete, the rock contains much more potash than 
would be expected from its intermediate position between the composition of a 
granite and a plionolitic rock. At the same time, there is as much pyrite and much 
more carbonates than in the altered latite-phonolite described from the Vindicator 
mine (p. 190). Some silica has been removed, while probably very little lime has been 
lost. 
METASOMATIC CHANGES IN SCHIST AND GNEISS. 
The gneiss is generally unaffected by hydrothermal metamorphism, but adjacent 
to veins some changes are often observed, as is well shown in specimens from the Abe 
Lincoln mine, consisting of a reddish gneiss with narrow seams coated by quartz 
combs and a little pyrite and calaverite. These seams are surrounded by a greenish 
bleached zone 1 or 2 inches wide, containing disseminated pyrite and looking almost 
as if it contained epidote. 
Under the microscope the veinlets show much adularia, chiefly deposited along 
the walls and covered by quartz. In the rock itself, which is originally composed of 
quartz, orthoclase, and biotite, the latter mineral is the only one which has suffered 
extensive alteration, though the orthoclase contains some disseminated sericite. 
The biotite is transformed to dolomite with much crystallized pyrite and some of the 
peculiar light-green mica described on page 77. The latter gives the peculiar yel- 
lowisli-green color to the rock. A little adularia, often with crystal form, is also 
present and in places it appears to replace biotite, a fact also observed in the granite 
ore. In the Bonanza King lode of the Midget mine pyrite, galena, and zinc blende 
sometimes replace fissured gneiss. Schist fragments occurring in breccia in the 
W. P. II. mine consist of quartz, sillimanite, muscovite, and magnetite. The only 
alteration observed is that the original magnetite is changed into pyrite and that 
irregular grains of calaverite have developed. 
METASOMATIC CHANGES IN BASIC DIKES. 
% 
The various dark-colored basic dikes classified as monchiquites, vogesites, and 
trachydolerites are rarely seen in fresh condition, but are apt to form soft, greenish 
gray sheets in which the products of common hydrometamorphism and hydrothermal 
metamorphism may be superimposed. Calcite develops very abundantly, especially 
along the platy structure so often seen, but this is a common thing in dikes of 
tills character and has not necessarily any connection with vein-forming agencies. 
Zeolites also form in many places, as do serpentine, epidote, talc, and iddingsite. 
